Embedding of fiber optic sensors in materials is a known technology to measure in-situ material deformations. In the ingress/egress strategy, one of the main problems is usually to find suitable methods for connecting this embedded fiber with an external fiber. Although several ingress/egress techniques exist, mainly based on feed through designs and external coupling, the resilience and strength of the coupling is not guaranteed.
Optical fiber sensors are normally spliced to a pigtail which connects the sensor with its read-out device. This typically needs some length of fiber, such as a few centimeters, starting from the edge of the composite. Composite structures are usually fabricated by moulding, in which cases embedding of optical fibers in composite laminates can be performed during fabrication. After fabrication and prior to use, trimming of the structure edges is often necessary. By trimming the edges, all fiber ingress points will be lost, and as a consequence splicing is very difficult or impossible to carry out when the fiber is broken at the edge of the composite.
A possible solution is avoiding cutting the fiber. For example, a connection may be provided in the waveguide embedded in the material during the fabrication. In case of composites, for example moulded composites, the solution is cumbersome. The embedded fiber in such system has restricted orientations and positions, and the production mould and established manufacture route needs to be adapted. The fiber is protected using Teflon or other types of tubing or Kapton foil. Sometimes the outcome of the fiber is protected with silicone. It is hardly possible to trim edges or remove mould burrs off the material. Other options include removing partially composite material, for example several layers on the side along the fiber, revealing a portion thereof, and then connecting the fiber in the revealed side. This is a cumbersome method, it requires a separate protective block for the fiber and the removed parts of the composite, and prior knowledge of the position of the fiber is necessary, or at least a reliable method to find it, which is not trivial in many cases.
Document DE102012020920A1 describes a method for connection of optical fibers used as optical sensors in e.g. automotive industry, involving embedding fiber in glass-fiber reinforced plastic, and turning front sides of ends of fibers towards each other. A portion of the embedding material, surrounding the fiber, is typically removed before an optical fiber ferrule and sleeve is used to align the embedded fiber with the external fiber. Finally, an arc is used to connect them. This method may damage the optical fiber and its cleave angle may introduce losses. These losses add up to the losses from splicing fibers, which take place especially in the usual case in which both fibers are different (e.g. one may be optimized for FBG sensing, while the other for signal transmission).
In the above discussed existing methods, there is no or very little room for repair or rework of a bad connection.